The invention concerns a security element in the form of a multi-layer film body which has a replication lacquer layer and a thin-film layer for producing a viewing angle-dependent colour shift effect by means of interference and in which a relief structure is shaped in the interface between the replication lacquer layer and the thin-film layer. The invention further concerns a security document having such a security element and a process for the production of such a security element.
Optically variable security elements are frequently used to impede and if possible prevent copying and misuse of documents or products. Thus optically variable security elements are frequently used for safeguarding documents, banknotes, credit cards, cash cards and the like. In that respect it is known for an optically variable element to be provided with a thin film which produces viewing angle-dependent colour shift effects by means of interference.
By way of example WO 01/03945 A1 describes a security product having a transparent substrate, to one side of which is applied a thin film which produces a perceptible colour shift in dependence on the viewing angle of the person viewing it. In that case the thin film comprises an absorption layer which is applied to the transparent substrate and a dielectric layer which is applied to the absorption layer. The absorption layer includes a material which is made up from one of the following materials or a combination of those materials: chromium, nickel, palladium, titanium, cobalt, iron, tungsten, molybdenum, iron oxide or carbon. The dielectric layer comprises one of the following materials or a combination of the following materials: silicon oxide, aluminium oxide, magnesium fluoride, aluminium fluoride, barium fluoride, calcium fluoride or lithium fluoride.
A diffraction pattern is applied on the opposite side of the transparent substrate in order further to increase the level of security against copying. That diffraction pattern acts as a diffraction grating so that for example the illusion of a three-dimensional image can be produced for the viewing person, by means of that two-dimensional pattern.
It is further proposed that the diffractive pattern is embossed on the side of the transparent substrate to which the thin film is also applied.
Those two configurations of an optically variable security element provide that the optical effects produced by the thin film layer and the optical effects produced by the diffractive pattern are superimposed at each location of the optically variable element and that therefore gives overall an optical effect which is composed of those two effects, that is to say for example the superimposition of a colour shift effect and a hologram.
In addition WO 02/00445 A1 describes an optically variable security element which comprises a plurality of mutually superposed layers and which has a thin film which produces the optical effect, as already described hereinbefore, of a viewing angle-dependent colour change. The optically variable security element further has a replication layer into which a relief structure is embossed. That relief structure produces a further optical effect, namely the diffraction effect which has already been described hereinbefore, by means of which holograms and the like can be displayed. In that respect, in terms of production procedure, firstly the thin film layer is applied to the replication layer and then the relief structure is applied by embossing.
In this connection, two procedures are proposed for decoupling the optical effects produced by the thin film and the relief structure: on the one hand it is proposed that an opaque layer is applied between the relief structure which produces a holographic image by means of diffraction and the thin film which produces a colour change. The relief structure is screened from the thin-film structure by means of that opaque layer. The second possibility involves arranging two or more layers of a substantially transparent medium between the relief structure which produces a holographic image by diffraction, and the thin film. Those layers can include one or more high-refractive layers and an adhesive layer. Reflection and thus the strength of the light in the region of the relief structure producing a holographic image are increased by means of those layers and the holographic image therefore stands out in relation to the colour shift effect of the thin film.